What causes green potatoes, and are they safe to eat?

Potatoes are specialized stems of the potato plant, and when exposed to light, will naturally turn green. The green is nothing more than chlorophyll, a harmless compound found in all green plants. However, when potato tubers turn green there is usually an increase in a glycoalkaloid compound called solanine. Tubers with a high concentration of solanine will taste bitter and can be harmful if eaten in large quantities. To be safe, it is best to not eat green potatoes. Consequently, it is important to store potatoes in the dark to prevent greening.